It is known that EPROMs can change their information content due to external influences--see "Woods" article in "Electronics" of Aug. 14, 1980, page 132. The EPROMs react in two possible ways: One memory cell, for example, may lose charge carriers and, due to that reason, change its state. It is also possible, however, that a memory cell accepts charge carriers due to external influences, which, likewise, may change the information content therein.
In order to insure that the information content in EPROMs remains, it has been proposed to read the information content in the EPROMs at recurring intervals, and then re-store the information content therein. Repeated storage of data in EPROMs, however, may detract from the storge characteristics of the EPROMs; it has been found that each new memory storage degrades the storage characteristics. Eventually, the storage cells, that is, the storage elements themselves, must be interchanged for new ones. Repetitive read-out and re-recording of memory content, whether necessary or not, and just in order to insure that the memory content remains the same, thus causes degradation of storage cells which, by themselves, would retain the memory content without change; the degradation affects goods as well as possibly marginally good, and poor or defective memory cells. Exchange of memory units, thus, will be necessary more frequently than would be the case if the information content in those EPROMs which are not defective is not re-recorded.